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doi: 10.3290/j.qi.a29146 1

QUINTESSENCE INTERNATIONAL

PROSTHODONTICS

Significance of crown shape in the replacement

of a central incisor with a single implant-

supported crown

Luca Gobbato, DDS1/Gianluca Paniz, DDS2/Fabio Mazzocco, DDS3/

Andrea Chierico, DDS4/Teppei Tsukiyama, DDS5/Paul A. Levi Jr, DMD6/

Arnold S. Weisgold, DDS7

Objective: When utilizing a single implant-supported crown to replace a central incisor,

understanding the final shape of the implant restoration is an important factor to help

achieve a successful esthetic outcome. In today’s dentistry, tooth shape is a critical factor

when dental implant prostheses are considered in the esthetic zone. The major esthetic

goal for this type of restoration is to achieve the closest possible symmetry with the adja-

cent tooth, both at the soft and at the hard tissue levels. The goal of this study was to

objectively analyze the significance of natural crown shape when replacing a central inci-

sor with a single implant-supported crown. Method and Materials: In this study, we inves-

tigated the shape of the crowns of maxillary central incisors in 60 individuals who

presented to our clinics with an untreatable central incisor. The presence of a dental dia-

stema, “black triangle,” presence or absence of gingival symmetry, and the presence or

absence of dental symmetry were recorded in the pre- and postoperative photographs.

Results: Out of 60 patients, 33.3% had triangular-shaped crowns, 16.6% square/tapered,

and 50% square-shaped crown form. After treatment was rendered, 65% of the triangular

group, 40% of the square/tapered group, and 13.3% of the square group required an

additional restoration on the adjacent central incisor in order to fulfill the esthetic needs of

the patients. Conclusion: Data analysis revealed that if there is a “black triangle,” a dia-

stema, or presence of dental or gingival asymmetry, an additional restoration on the adja-

cent central incisor is often required in order to fulfill esthetic goals. The additional

restoration is highly recommended in situations with a triangular crown shape, while it is

suggested in cases of square/tapered and square tooth shapes in the presence of a den-

tal diastema. (doi: 10.3290/j.qi.a29146)

Key words: biotype, black triangle, dental implant, esthetics, interproximal papilla, tooth shape

1Clinical Instructor, Department of Oral Medicine, Infection &

Immunity, Harvard School of Dental Medicine, Boston, MA, USA.

2Adjunct Assistant Professor, Department of Prosthodontics and

Operative Dentistry, Tufts University School of Dental Medicine,

Boston, MA, USA.

3Private Practice, Padova, Italy.

4Private Practice, San Pietro in Cariano, Verona, Italy.

5Private Practice, Japan.

6Clinical Associate Professor, Department of Periodontology,

Tufts University School of Dental Medicine, Boston, MA, USA.

7Adjunct Professor, Department of Periodontics, University of

Pennsylvania School of Dental Medicine, Pennsylvania, PA,

USA.

Correspondence: Dr Luca Gobbato, Harvard School of Dental

Medicine, Department of Oral Medicine, Infection & Immunity,

190 Longwood Avenue, Boston, MA 02115. Email: Luca_

[email protected]

Since implant dentistry has become a com-

mon treatment for replacing missing teeth,

dentists have been trying to mimic natural

tooth morphology in order to achieve a

proper functional result and to fulfill the

patient’s esthetic desires.1-3

The criteria for implant success4,5 pub-

lished in 1986 have since been profoundly

reviewed, and what was considered the

standard of care 20 years ago is not neces-

sarily valid today. The criteria then for an

implant included osseointegration and the

possibility for an optimal restoration.

Today achieving an acceptable treat-

ment outcome in implant dentistry no longer

depends on simply achieving osseointegra-

tion or being able to restore an implant.6

2 doi: 10.3290/j.qi.a29146

QUINTESSENCE INTERNATIONALGobbato et al

Clinical success now demands that, at

the least, an esthetically pleasing result as

determined by the patient and the clinician

should be the therapeutic end point. This

means establishing an ideal balance

between the “pink” and the “white” compo-

nent of the dentogingival complex, as dis-

cussed by Buser and coworkers when

defining the white esthetic score (WES) and

the pink esthetic score (PES).7

Esthetic dentistry involves both psycho-

analytic and social behaviors. The treat-

ment of someone’s smile is a delicate and

extremely important result of treatment, not

only because the smile is one of the first

things people see, but also because an

attractive smile improves overall appear-

ance and helps provide an individual with

self-confidence.

In an ideal situation, the operator should

follow documented prosthetic guidelines in

order to achieve the optimal esthetic goals

including: symmetry, harmony, unity with

variety, and an appropriate tooth shade.2,8

Compromising one of these factors will ulti-

mately result in an unfavorable result espe-

cially when dealing with the maxillary cen-

tral incisors.9

Despite the biological and technical

advances recently made in the field of

implant dentistry, there are still a number of

clinical scenarios where clinicians may

encounter certain limitations in their ability

to achieve the ideal esthetic outcome. The

widespread use of dental implants as the

therapy of choice to replace missing teeth

means that clinicians involved in patient

care must foresee these limitations and

understand the final outcome of the treat-

ment as much as possible before therapy

begins. Addressing the specific risk indica-

tors during the diagnostic phase will help

therapists identify when additional therapy

may be required or when an outcome that

may not match dentist or patient expecta-

tions is likely to occur.

To help categorize the difficulty level of a

given treatment, in 2007 the International

Team for Implantology (ITI) formalized a sys-

tem of classification for dental implant pro-

cedures to support clinicians at every level

of expertise and experience.10 This publica-

tion is based on the debate and findings of

an ITI Consensus Conference attended by a

multidisciplinary group of 28 clinicians that

was held in Mallorca in March 2007. It pro-

vides guidelines to a broad variety of implant

situations for both restorative and surgical

cases, which are classified according to

three categories: straightforward (S),

advanced (A), and complex (C) (SAC).

Acknowledging the challenging clinical con-

ditions often present in the anterior maxilla

such as lip line at smile, number of missing

teeth, bone quality and quantity, gingival

biotype, and the tooth shape can profoundly

influence the degree of treatment risk.10

When utilizing a single implant-sup-

ported crown to replace a central incisor,

understanding the final shape of the implant

restoration is an important factor to achieve

a successful esthetic outcome. Tooth shape

is a critical factor when dental implant pros-

theses are treatment planned in the esthetic

zone.9,11,12 The major esthetic goal for this

type of restoration is to achieve the closest

possible symmetry with the adjacent tooth

at the soft and at the hard tissue levels.

The shape of the missing and adjacent

teeth profoundly influence the degree of

esthetic success associated with implant-

supported restorations in the esthetic

zone.13 According to previous literature,

with the esthetic outcome strongly influ-

enced by the final gingival architecture,

successful esthetic results can be enhanced

by the presence of square teeth.13

In a previous study the authors identified

a range of measurements in which it is pos-

sible to categorize the form of the crown of

the maxillary central incisors into three dif-

ferent groups: triangular, square, and

square/tapered.14

The purpose of this paper is to verify,

through a retrospective analysis, if tooth

shape can dictate decision making as to

whether or not an adjacent natural central

incisor will require an additional restorative

procedure to enhance the final esthetic

result.

METHOD AND MATERIALS

For this retrospective analysis, consecutive

patients with a central incisor treated by the

authors with a single implant restoration

between June 2006 and July 2011 were

doi: 10.3290/j.qi.a29146 3


selected. The study population consisted of

60 adults between 22 and 37 years of age,

in good health, and with growth and devel-

opment completed.

As inclusion criteria, preoperative and

postoperative anterior photos were required.

During the recruitment of the patients the

following exclusion criteria were employed:

presence of a prosthetic restoration on the

maxillary central incisor that was not going

to be replaced, a history of tooth trauma

causing a change of shape of the incisors,

presence of incisal attrition, abrasion or ero-

sion that reached the dentin, presence of

crowding or tooth rotation, or evidence of

altered passive eruption.

Clinical measurements

In the preoperative photographs the shapes

of the maxillary central incisors were evalu-

ated and each central incisor was classified

as triangular, square/tapered, or square

(Fig 1).14

Regarding the contact surface (CS)

length/crown length (CL) ratio, if CS is less

than 43% of CL, the tooth is triangular in

shape; if CS is more than 57% of CL, the

tooth shape is square.14

The presence of a dental diastema,

“black triangle,”12 presence or absence of

gingival symmetry, and the presence or

absence of dental symmetry was recorded

by preoperative photography. The presence

of a diastema was identified as a space in

between the two central incisors. The pres-

ence of a “black triangle” was identified as

a space that had developed when the inter-

proximal papilla receded apically, resulting

in the appearance of a dark triangle.

Gingival and dental symmetry was con-

sidered achieved when the exact corre-

spondence of form and constituent configu-

ration on opposite sides of a dividing center

axis was present.

The same analysis was performed on

the postoperative photograph (Fig 2). All

patients included in the study did not have

“black triangles” or diastemas at the end of

treatment, and presented with gingival and

dental symmetry. The initial and final condi-

tion and modifications were correlated to

the treatment rendered.

The additional restorations were subdi-

vided into:

additional restoration (composite resin

restorations or porcelain laminate

veneers; no tooth preparation was per-

formed)

partial or full coverage restoration (por-

celain-fused-to-metal or all-ceramic res-

torations performed with tooth

preparation).

The variables considered on the contralat-

eral central incisor were:

no treatment performed

additional restoration (composite resin

restorations or porcelain laminate

veneers; no tooth preparation was per-

formed)

Fig 1 Classification for the tooth shape of central incisors. Regarding the CS/CL ratio (R), if CS is less than 43% of CL, the tooth is triangular in shape; if CS is more than 57% of CL, the tooth shape is square.

Triangular Shape Squared-Tapered Shape

Classification for Tooth Shape of Central Incisor

Contact Surface Length (CS) / Crown Length (CL) Ratio

Square Shape

57% < R43%< R <57%R < 43%

4 doi: 10.3290/j.qi.a29146


partial or full coverage restoration (por-

celain-fused-to-metal or all-ceramic full-

coverage restoration performed with

tooth preparation).

Data analysis

A descriptive analysis was performed in

order to demonstrate the correlation

between the need for an additional restor-

ation and the achievement of the esthetic

result. The P value was set at .05.

Sixty patients were selected to be part of

this retrospective analysis. We included

only the cases where the esthetic outcomes

were completely fulfilled (no diastema, no

“black triangle,” and dental and gingival

symmetry was achieved). Out of 60 patients,

before the treatment was rendered 20

exhibited triangular tooth forms, 10 showed

square/tapered tooth forms, and 30 had

square tooth forms.

RESULTS

In order to achieve an acceptable esthetic

result (no diastema, no “black triangle,” and

dental and gingival symmetry), after implant

insertion in the ideal three-dimensional

position (Fig 3) 65% of the teeth that were

initially triangular in shape required an addi-

tional restoration on the adjacent central

incisor, 40% of the square/tapered group

required an additional restoration, and only

13.3% of the square group required an

additional restoration on the adjacent cen-

tral incisor (Table 1, Figs 3 and 4).

Considering only the 21 patients who

required an additional restoration:

13 subjects (61.9%) were treated with

full coverage restorations with tooth

preparation (all originally exhibited trian-

gular tooth forms)

8 subjects (38.1%) were treated with

additive composite resin restorations (4

originally exhibited square tooth shape

and 4 exhibited square/tapered;

Table 2).

As is shown in Table 3, the teeth that

required an additional restoration had their

original form changed from triangular or

square/tapered to a square tooth shape.

A student t test was performed to show

statistically significant differences among

different groups of patients. The student t

test was used to compare the percentage

of triangular teeth that required an addi-

tional restoration with the percentage of

subjects belonging to the square-shaped

group that required an additional restora-

tion (statistically significant, P < .05).

Fig 2 Significance of crown shape in the replacement of a central incisor with a single implant-supported crown. The diagrams represent the classification of shape:14 when the CS/CL ratio is less than 43%, a maxillary central incisor is categorized as triangular; between 43% and 57%, the tooth is defined as square/tapered; more than 57%, the tooth is defined as square. (a) Anterior view of an untreatable maxillary right triangular central incisor. (b) Anterior view of an untreatable maxillary right square/tapered central incisor. (c) Anterior view of an untreatable maxillary right square central incisor.

57% < R

Square

43%< R <57%

Square Tapered

R < 43%

Triangular

Contact Length / Crown Height Ratio

a b c

doi: 10.3290/j.qi.a29146 5


Table 3 The change in the shape of teeth that required an additional restoration

Initial shape

Final shape

Triangular Square/tapered Square Total

n (%) n (%) n (%) n (%)

Triangular 0 (0.0) 0 (0.0) 13 (100.0) 13 (100.0)

Square/tapered 0 (0.0) 0 (0.0) 4 (100.0) 4 (100.0)

Square 0 (0.0) 0 (0.0) 4 (100.0) 4 (100.0)

Total 0 (0.0) 0 (0.0) 21 (100.0) 21 (100.0)

Table 2 Effect of initial tooth shape

on type of additional restor-

ation required

Initial shape

Type of additional restoration

Crown Additive

n (%) n (%)

Triangular 13 (100.0) 0 (0.0)

Square/tapered 0 (0.0) 4 (100.0)

Square 0 (0.0) 4 (100.0)

Total 13 (61.9) 8 (38.1)

Table 1 Effect of initial tooth shape

on the need for an additional

restoration

Initial shape

Requiring additional restoration

Yes No

n (%) n (%)

Triangular 13 (65.0) 7 (35.0)

Square/tapered 4 (40.0) 6 (60.0)

Square 4 (13.3) 26 (86.7)

Total 21 (35.0) 39 (65.0)

Fig 3 (a to c) Implant placement was performed in an ideal three-dimensional position.

Fig 4 In order to achieve acceptable esthetic results, 65% of the teeth that were initially triangular in shape required an additional restoration on the adjacent central incisor, 40% of the square/tapered group required an additional restoration, and 13.3% of the square group required an additional restoration on the adjacent central incisor. (a) Final restoration on maxillary right central incisor. A porcelain laminate veneer was added on the maxillary left central incisor in order to modify the tooth shape from triangular to square. (b) Final restoration on maxillary right central incisor. A porcelain laminate veneer with no tooth preparation was added on the maxillary left central incisor in order to close the initial diastema. (c) The final restoration on maxillary right central incisor was placed, leaving an unaltered maxillary left central incisor.

a

a

b

b

c

c

6 doi: 10.3290/j.qi.a29146


DISCUSSION

The use of a single dental implant for the

replacement of a maxillary central incisor is

an esthetically challenging situation from

both a surgical and a restorative perspec-

tive. The three-dimensional position of the

fixture,9,15 the gingival architecture,16,17 the

periodontal biotype,16-18 the residual ridge

dimension, and the shape and shade of the

prosthesis will influence the overall appear-

ance. The dental shape is determined by

the ratio between CL and CS. Therefore a

triangular-shaped tooth will have a shorter

(apico-occlusal) CS when compared with a

square tooth. With triangular-shaped teeth,

the shorter CS is accompanied by an apico-

incisally longer papilla than is seen with a

square tooth. As noted by Tarnow et al,11

when the vertical distance between the api-

cal extent of the contact point and the crest

of the interproximal bone is 5 mm or less the

papilla is almost always present. Conversely,

when the distance is 7 mm or more the

papilla is usually missing. Thus, in the pres-

ence of a triangular-shaped tooth the papilla

is most likely missing or at least not filling all

the space required to reach the most apical

portion of the CS. Within minutes following

the extraction of one of the two central inci-

sors the height of the papilla will lose verti-

cal dimension,19 creating difficulty for an

esthetically satisfactory restoration. As it is

not possible to predictably regenerate a

papilla,20 an additional restoration is

required on the adjacent tooth in order to

create symmetry of both central incisors,

which will lengthen the contact area apico-

occlusally,21 shorten the apico-occlusal

length of the papilla, and shorten the dis-

tance from the base of the contact point to

the crest of the interproximal bone. This will

help eliminate the “black triangle” caused

by deficient papillary length.

If a square/tapered central incisor is

being replaced by an implant, if required an

adjacent additional restoration can often be

performed with composite resin bonding,

whereas if the central incisor is triangular,

the adjacent natural tooth will most likely

require a veneer or a full crown in order to

achieve symmetry and provide an embra-

sure space filled with a papilla.

Although orthodontic driven eruption of

the adjacent central incisor may improve

the height of the papilla,22,23 an additional

restoration will most likely be needed any-

way and the time of the treatment would be

significantly increased.

In conclusion, there are no universal

guidelines for clinicians to follow in creating

greater uniformity and a predictable esthetic

smile, including ideal papilla heights: the

more perfect esthetics that patients

demand, the more clinicians attempt to

achieve perfection and ideal symmetry. Our

goal is to fulfill our patient’s expectations.

Since esthetics is emotionally driven by

each patient, the expectation and the con-

cept of esthetics vary from patient to patient.

CONCLUSION

Data analysis revealed that if a “black tri-

angle,” diastema, or dental or gingival

asymmetry is present, an additional restor-

ation on the adjacent central incisor is usu-

ally required in order to fulfill the patient’s

esthetic goals. In the presence of a triangu-

lar tooth shape and a patient with high

esthetic anticipation, a full coverage restor-

ation may be more suitable; whereas com-

posite resin bonding might be the suggested

restoration in situations where there are

square/tapered and square tooth forms in

the presence of a dental diastema.

In summary, when working with a patient

who requires treatment in the esthetic zone,

understanding their expectations will influ-

ence treatment planning choices. Other

important determinants such as tooth shape

are indicators for understanding the com-

plexity of the therapy, and will help to dic-

tate the additional treatment that might be

required in order to fulfill patient’s expecta-

tions.

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